The current state of the art in roof deck construction for commercial, residential, and industrial buildings is controlled very rigidly in the United States. This control has been generated by legislation primarily at the state and local levels through the adoption of state and local building codes which set forth very specific performance factors. All roof decks, in order to comply with these regulations, must meet established performance standards. In general, these performance standards are divided into two broad areas: (1) Sloped roof decks, generally 30 degrees or greater from horizontal and (2) Flat roof decks, 0 degrees to 30 degrees slope from horizontal.
The performance standards for flat roof deck construction vary slightly from area to area but generally conform to the following:
1. Vertical Load Strength: A roof deck must be able to carry a total load consisting of dead load plus live load and not exceed legislated design or performance values for the materials being utilized in the roof deck assembly.
Example: Conventional steel roof decks manufactured from 50,000 to 60,000 psi steel must not be stressed under working conditions beyond a flexural tensile stress of 20,000 psi.
2. Live Load Deflections: While supporting the designed dead load (weight of steel deck, built-up roof and insulation) the roof deck must not deflect under live load application more than 1/240th of the distance between the support members.
Example: A roof deck supported by members 6'0" on center must not deflect more than 6'0".times.12 in./ft..times.1/240 equal 0.30" under live load application. Live loads will vary in different climate areas from 20 pounds per sq. ft. to 60 lbs./sq. ft., depending upon weather conditions.
3. Wind Up-Lift Resistance: While not at this time in complete use by all code bodies, this performance requirement is being adopted fairly rapidly and currently is in use in many areas. Under wind loadings from storms, hurricanes, etc., the roof deck must resist negative and positive pressures applied to it and remain structurally serviceable. Performance values for this standard vary depending upon geographical areas, but in general, range from 30 psf uplift resistance (equivalent of 100 mph winds) to 90 psf uplift resistance (equivalent of 188 mph winds).
Heretofore steel roof deck assemblies have utilized sections formed from mild steel in patterns normally referred to as "Type A", "Type B", "Type AB", and the like. The common feature of the sections is a wide flat surface element, formed between stiffening ribs that provide the stiffness and strength to the section. The steel sections, supported by purlins, have been designed heretofore to meet strength requirements specified by building codes. The flat surfaces has been employed to provide a supporting surface for one or more layers of sheet material comprising a single board serving to insulate and provide a surface to which waterproof covering was attached.
A typical "Type A" section, for example, provides a flat portion of approximately 51/2 inches wide between 11/2 inch deep stiffening ribs that are spaced six inches apart. The "Type B, AB" and other sections are similar in profile to a Type A section except that the flat portions between stiffening ribs is progressively reduced in width to create a closer spacing of the stiffening ribs, increasing the load capacity for a given span. However, the width of rib openings on the top surface of the sheet, for example of a Type B section is greater than that of a Type A section.
The most efficient light gauge steel sections from a strength standpoint are those that have the greatest number of stiffening ribs per unit of width; the ultimate, being the symmetrical rib pattern sections, which have an equal distribution of steel above and below a neutral axis lying in a plane passing through the center of the sheet and disposed parallel with upper and lower surfaces of the sheet.
The symmetrically corrugated sheet section is not new to the construction industry and has been utilized for many years as siding and roofing. However, the symmetrically corrugated configuration has not been used in flat roof dry installed roof deck construction because it does not comply with the required performance standards when installed in conventional manner. While theoretically being able to support design loads, in practical use the section bends and distorts under loading, therefore destroying its load carrying capabilities. The sections exhibit poor flexural capabilities in deflection and therefore cannot satisfy the deflection requirements specified by building codes.
It may be mandatory in some cases, or desirable in other cases, for economy reasons, to utilize the roof deck assembly as a structural diaphragm to reinforce a building against lateral loads created by earthquake shocks (sesmic), explosion forces or wind. In such application, the roof deck assembly is considered to be the plate web of a girder oriented in a horizontal plane with the perimeter members of the building serving as the compression and tension chords of the girder.
The diaphragm (plate web) strength of a given roof deck assembly is evaluated in terms of its ability to transfer diagonal tension stresses, which involves consideration of the shear resistance of the assembly, and in-plane deflection (referred to as "diaphragm deflection"), which is governed to a large extent by the "diaphragm stiffness" of the steel panel sections that are utilized. Diaphragm stiffness is related to the ability of the steel panel sections to resist distortion under load.
It is generally known that an "ideal" diaphragm would consist of a thin plane sheet or membrane attached to a structure in such a way (at the support level) that it can resist shear forces through diagonal tension field action. Heretofore it has not been possible, however, for a steel roof deck assembly to function as an "ideal" diaphragm because to satisfy their purpose, roof deck assemblies are also required to support vertically imposed loads which requires rib construction hereinbefore described. Accordingly, the diaphragm stiffness that a given steel panel section can provide depends on the orientation of the steel in the section to the stress plane, which is located at the immediate top of the supporting purlins. In this respect, flat profile steel panel configurations wherein most of the steel is elevated above the support level (the stress plane) have less diaphragm stiffness than sections that provide more steel nearer to the stress plane such as a symmetrical rib pattern.
Since the flexural strength of a steel panel section is, to a large degree, a function of the depth of the section, it is naturally opposed to the reduction of depth (approaching a thin plane of steel) that contributes to diaphragm strength. The most efficient roof deck assemblies, from the standpoint of diaphragm strength, are those that can provide adequate flexural strength, utilizing steel sections with the maximum degree of effective steel in the diaphragm stress plane. Diaphragm stiffness increases proportionally to increases in the yield strength of the steel that is utilized, hence, steel sections made of high tensile steel are more effective than those made of mild steel.
Heavy gauge, mild steel (for example, 22 gauge, 20 gauge and 18 gauge with a stress limit of 20,000 lbs. per square inch) is generally employed in the manufacture of Type A and similar flat profile sections. This has been due to the fact that heavier gauges are necessary to satisfy the minimum steel thickness to element-width ratios that govern the design of light gauge steel sections. On the other hand, the symmetrical rib pattern sections have smaller unit-width elements and hence can utilize the more effective high tensile strength steel in lighter gauges providing greater working strength per pound of steel.
Asphalt built-up roof coverings usually consist of several layers of asphalt-saturated felt with a continuous layer of hot-mopped asphalt between the layers of felt. The top layer of such a roof covering may consist of a hot mopping of asphalt or coal tar pitch only, a top pouring of hot asphalt with slag or gravel embedded therein, or a mineral-surfaced cap sheet embedded in a hot mopping of asphalt.
Built-up roofs cannot generally be applied directly to steel roof deck sections and consequently an underlayment of substrate material has heretofore been installed after the steel roof deck sections have been secured in place. The single sheet of underlayment material has heretofore been generally referred to as "rigid roof insulation board". However, the insulating efficiency of the rigid board insulation is generally directly related to the density of the materials of which it is constructed, lighter density materials providing proportionally better insulation for a given thickness. Strength characteristics are inversely related to reductions in density. Accordingly, the lighter the density, the less the strength. Since "rigid insulation board" has heretofore been used over steel decks to provide a suitable base for roofing as well as insulation, the board had to be manufactured in densities that would compromise the minimum requirements for strength versus insulation values. Typical of compromised situations, the "rigid insulation boards" have been made to be adequate, but under the circumstances could not be fully efficient in the performance of either function, i.e., providing thermal insulation and strength.
Because of the inherent low strength of "rigid insulation boards" constructed of relatively low density material to provide the desired thermal insulation, the boards had to be fully supported because sufficient strength was not provided to create a structural bridge over wide voids in the surface of the steel roof deck section supporting the insulation board.
The low density material found in a single sheet to serve the dual role of a roof substrate member and an insulating member did not have sufficient internal strength to hold screws or other conventional forms of mechanical fasteners. Consequently, the Type A and similar flat profile sections of steel roof deck sections, having narrow rib openings on the surface, were necessary to provide the support to the board and to provide a sufficiently large contact area to facilitate attachment with hot asphalt or other types of adhesives. Therefore, the less efficient flat profile steel sections have heretofore been employed in lieu of the more efficient symmetrical rib pattern sections which have wide rib openings on the surface.